Abstract
In order to improve our knowledge of the weathering response of tempera paints exposed to an industrial atmosphere, azurite- and malachite-based paint mock-ups prepared with either rabbit glue or egg yolk binders were artificially aged in an SO2 rich atmosphere. The aim was to identify the different alteration mechanisms and forms of degradation in the paints by observing their physical (color, gloss, reflectance, and roughness), mineralogical, chemical, and micro-textural characteristics. Superficial physical changes were evaluated by stereomicroscopy, spectrophotometry, gloss measurement, hyperspectral imaging, and roughness measurements. Chemical and mineralogical changes were evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy with microanalysis (SEM-EDS), which was also used to evaluate the micro-texture of the paints. The differences between the fresh temperas were due mainly to the binder (egg yolk or rabbit glue) used in the paint mixture, which also played a crucial role in the different deterioration patterns in the artificially aged paint mock-ups. Thus, the egg yolk-based paints remained physically quite intact after SO2 exposure, although they suffered more significant chemical degradation, above all in the form of copious precipitation of Cu and Ca-rich sulfate salts and the subsequent yellowing of the egg yolk binder. The SO2 aged rabbit glue-based mock-ups showed microscopically important crack formation and binder loss and fewer sulfated salts precipitated on the surface of the paints.
Highlights
In order to design preventive conservation strategies to protect polychrome artworks and historical paintings, paint mock-ups are used to evaluate the damage caused by different agents such as relative humidity, irradiation, and atmospheric gases including pollutants among others [1,2,3,4,5,6,7,8,9,10]
The AZ mock-ups made with egg yolk had a slightly greener color than their counterparts made with rabbit glue; the azurite standard (AZ-ST) mock-ups made with egg yolk and rabbit glue had a similar color
The results obtained from this research allowed us to reach the following conclusions: The fresh azurite- and malachite-based tempera mock-ups prepared with the two binders, i.e., egg yolk or rabbit glue, had certain physical-chemical differences that affected their capacity to withstand the accelerated aging test in which they were exposed to SO2
Summary
In order to design preventive conservation strategies to protect polychrome artworks and historical paintings, paint mock-ups are used to evaluate the damage caused by different agents such as relative humidity, (solar/artificial) irradiation, and atmospheric gases including pollutants (ozone, nitrogen oxides, carbon dioxide, and sulfur dioxide) among others [1,2,3,4,5,6,7,8,9,10]. Minerals 2020, 10, 424 materials (stones, bricks, mortars, etc.) used in historic monuments are exposed to SO2 , oxidation occurs, producing one of the most serious forms of alteration, i.e., black gypsum crusts. These processes have been widely studied in both carbonate sedimentary rocks and granites [15,16,17], and it seems likely that this gas would have even more harmful effects on more sensitive artistic materials such as tempera paints. The interaction between SO2 and tempera paints should be investigated in order to develop preventive conservation strategies to protect these artworks
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